Cancer stem cells may constitute a rare population of tumor cells that has the capacity to self renew and to give rise to the bulk of transformed cells. Despite its potential importance, cancer stem cells alone cannot maintain tumor tissue. Tumor associated stromal cells including endothelial cells constitute a significant fraction of tumor tissue that are critical for its growth and expansion. Neovascularization is a fundamental process that is intimately integrated with a stem cell program leading to tissue generation in both physiologic and pathologic processes. A number of studies have demonstrated that the vast majority of endothelial cells undergoing neovascularization are derived from non-hematopoietic rather than bone marrow derived hematopoietic stem cells. A significant fraction of tumor-associated stromal cells may therefore be derived from progenitors that reside in peripheral tissue. The skin, the largest organ, contains a number of stem cells such as keratinocyte stem cells that participate in its maintenance and repair. In addition, we hypothesize that as yet poorly characterized resident stem cells in the skin contribute to vessels and the stroma during wound healing and tumor expansion. The broad long term objective of the study is to identify and characterize resident cells in the skin that give rise to tumor-associated stromal cells including endothelial cells. Towards this goal, we propose 3 specific aims. In specific aim 1, we will define stromal cell subsets using a combination of lineage and stem cell markers, determine their numbers and spatial distribution, and confirm their origin in both tumor and in adjacent skin tissue. In specific aim 2, we will isolate stromal cell subsets and examine their lineage relationships based on gene expression patterns. In specific aim 3, we will determine the frequency of stromal cell subsets to form vascular structures in vivo. The identification of resident stromal stem cells will reveal new targets for inhibiting tumor growth, provide a foundation for a more detailed understanding of tumor and adult stem cell biology, and have an important impact on promoting wound healing and tissue engineering. PUBLIC HEALTH RELEVANCE: The identification of resident stromal stem cells will reveal new targets for inhibiting tumor growth, provide a foundation for a more detailed understanding of tumor and adult stem cell biology, and have an important impact on promoting wound healing and tissue engineering.